Put your negative probe to the negative post on your battery. You then place the positive probe in the neck of your radiator.
0.2 V to 0.5 V - antifreeze is still good
0.5 V to 0.7 V - antifreeze is borderline
0.7 V or greater - antifreeze is unacceptable.

Lower voltage reading means less lead,benzene, and metals have contaminated the antifreeze.